This invention relates to a laser Doppler velocimeter for the measurement of the flow of a liquid such as blood or of a gas or powder.
Measurement of the velocity of blood flow has now become a potential diagonostic method for detecting diseased portions in the circulatory system represented by vascular stenosis and vascular thrombosis or for investigating vital reactions occurring in consequence of administration of treatments and medicines. The apparatuses heretofore known in the art to render this measurement feasible mainly include electromagnetic flowmeters, ultrasonic Doppler velocimeters and laser Doppler velocimeters (hereinafter called "LDV"). Because of a common advantage that they provide quick measurement, require no incursion, cause no pain to the subject, and so on, such apparatuses are finding increasing acceptance in clinical use. While all the conventional apparatuses are effective in the measurement or blood flows at relatively high speeds, they exhibit inferior resolving power in measuring blood flows at low speeds. In extreme cases, they are totally incapable of providing required measurement.
Among these conventional apparatuses, the LDV is held to be particularly suitable for medical applications because the frequency of the laser beam for producing the Doppler beat frequency is relatively high and proportionally easy to detect and, moreover, because the laser beam can be constricted to a diameter small enough for the beam to be accurately focused on specific fine spots such as in the capillaries.
The LDV provides measurement of the velocity of a given blood flow by directing the laser beam onto the blood vessel and detecting the time-course change in the intensity (Doppler beats) of the forwardscattered portion of the beam, the change in intensity being generated in consequence of the Doppler shift caused by the motion of the red blood cells. Several methods have been proposed to date for performing this measurement. One proposal has been made by P. Buchhave, "Laser Doppler Velocimeter with Variable Optical Frequency Shift" (Optics and Laser Technology, Feb. 1975, pp. 11-16). This particular method involves beating a variable, local-oscillated frequency with the Doppler shifted signal to determine the shift frequency. By this method, which is essentially heterodyne system, it is possible to detect a high shifted frequency in the range of from 10 KHz to 50 MHz. Moving particles producing a shifted frequency within this range have a relatively high flow velocity and this method permits effective measurement of the velocity of flow of particles which are flowing at such a high speed. It is, however, incapable of measuring the velocity of moving particles passing through an extremely small region and producing a lower shifted frequency. This is because, during the power spectrum analysis of the beat frequency of the scattered light, the pedestal component is invariably in the extremely low frequency band overlapping the low Doppler beat frequency produced as a result of the slow motion of the particles so that the two frequencies cannot be discriminated from each other.
To overcome this difficulty, H. Mishina et al have proposed in "A Laser Doppler Microscope" (Optics and Laser Tech., June 1976, pp 121-127) a method for eliminating the pedestal component affecting the desired beat signal by means of an optical system using light polarization or a method resorting to an expensive electronic tracking filter. As indicated in the report, however, the former method using the light polarization necessitates a complicated optical system and the latter method using the electron tracking filter has a disadvantage that successful tracking becomes difficult unless beat signals are detected continuously whereas beat signals of a low frequency are in fact obtained in bursts in the measurement of moving particles passing through an extremely small region. The two methods, accordingly, have not been practicable.
The present inventors previously proposed a method for effectively eliminating the pedestal component as an extraneous component as reported in a Japanese Journal titled "Oyo Butsuri, Vol. 48, No. 2, pp. 175(83)-178(86)." Because the pedestal component corresponds to the sum of the outputs which would be derived from the photosensor by separately applying two beams to a given specimen, this method selectively extracts the Doppler frequency by deducting the aforesaid sum of the outputs from the output derived by simultaneous use of the two beams. This method, therefore, requires three kinds of data obtained by separate use and simultaneous use of the two beams upon the specimen. It has the disadvantage that it entails time-consuming work for data collection.
Other literature dealing with the LDV includes Japanese Patent Publications No. 63634/1980 and No. 75668/1980. Another article dealing with the LDV has been written by J. Oldengarm under the title "Development of Rotating Diffraction Gratings and Their Use in Laser Anemometry" (Optics and Laser Tech., Apr. 1977, pp. 69-71). This concerns adoption of a diffraction grating as a combination beam splitter and frequency shifter.
As described above, when the beat frequency resulting from the Doppler shift is manifested on the power spectrum, the pedestal component which varies with the distance over which the particles move through the zone irradiated by the laser beam is inevitably within an extremely low frequency band. No practical and reliable technique has been developed for providing effective elimination of the pedestal component from the desired beat signal. In the measurement of a low velocity of liquid flow, when the beat frequency approaches or overlaps with the pedestal component band, it becomes difficult or even impossible to discriminate the two frequencies from each other.
The conventional methods for the measurement of blood flow, even including the LDV capable of highly efficient measurement, have proved unsuitable for the measurement of low velocities of blood flow as described above. In these circumstances, strong need has been felt for improvements in the LDV as a medical apparatus useful in investigating various disorders of the circulatory system.